Groundwater Science - Northeastern Illinois, Illinois State Water Survey

Groundwater Science

Northeastern Illinois

As early as the 1860s, wells were being drilled into what was to become the most heavily developed aquifer system of the region, the deep bedrock Cambrian-Ordovician aquifers. Despite the proximity of Lake Michigan, the presence of this regional aquifer system beneath the whole of northeastern Illinois contributed greatly to the industrial and population expansion of the area.  Wells reaching this important aquifer system are typically 800 to 1,500 feet deep. Shallower wells tap and use overlying formations that are not so widespread.  

As the industry and population grew in the Chicago region, withdrawals exceeded the region's 65 million gallon per day (mgd) estimated sustained yield for the Cambrian-Ordovician aquifer system. By 1979, pumpage from this deep bedrock aquifer in the eight-county area in northeastern Illinois reached an all-time high of 182.9 mgd, nearly triple the estimated sustained yield of the aquifer system. By 1980, continued heavy pumpage had caused the potentiometric level of the deep bedrock aquifers at Chicago to decline more than 850 feet. Until Lake Michigan water became available to the collar counties in the early 1980s, pumpage in excess of the sustained yield had occurred every year since the late 1950s. Withdrawals from the deep bedrock aquifers are currently very near the estimated sustainable yield. As a result of Lake Michigan diversions and consequent reductions in withdrawals from the deep bedrock, recovery of water levels has been observed in some areas. However, continued heavy pumpage persists in the Joliet area corresponds with the deepest groundwater levels. 

Groundwater Quality  As a general rule, the water in the deep bedrock aquifers tends to be more mineralized than shallow aquifers, but it varies from location to location. The potential for contamination by vertical migration of chemicals from the land surface is very low; however, other contaminant pathways, such as abandoned wells, can pose a threat to the groundwater quality in the deep bedrock. The Cambrian-Ordovician is also known to contain high concentrations of naturally occurring barium and radium. 

Because of increases in water demand due to rapid growth in the region, the shallow bedrock and overlying sand and gravel aquifers will need to be utilized more. In a study funded by the Illinois Groundwater Consortium (IGC), ISWS researchers have examined historical water quality data to determine if there are temporal changes in water quality in these shallow unconfined aquifers. The data indicate that concentrations of several major ions, especially chloride, and total dissolved solids have increased in the shallow aquifers in the last 20 years, especially in the shallowest wells (< 100 ft). A poster on this study was recently presented at the Geological Society of America annual meeting in Seattle. Proceedings papers from IGC meetings in 2001 - 2003 are also available for viewing:

Lake Michigan Diversion  U.S. Supreme Court decrees limit Illinois to diverting an average of 3,200 cubic feet per second (cfs) from Lake Michigan. Illinois exceeded this limit during 11 of the 15 years from 1981 through 1995; the State diversion averaged 3,197 cfs during 1995. Essentially, Lake Michigan water is already fully allocated.

Increasing Demands  The Northeastern Illinois Planning Commission projects the population of the six northeastern counties to grow by 25 percent by 2020 and that of the outer collar counties to increase by 70 to 100 percent by 2020. Deregulation of the power industry, low natural gas prices, and the ever-increasing need for electrical power have combined in an explosion of proposals for peaking power plants in northeastern Illinois, placing yet additional demands for water on already scarce resources.

Science for Water Resources Management  As a result of aggressive resource evaluation and management, Illinois has reduced Cambrian-Ordovician pumpage in the Chicago region to near the practical sustained yield of the aquifer. However, this aquifer system and Lake Michigan are now at or near their sustainable or legally mandated limits and cannot be relied upon as significant sources of additional water for the region. The most viable water source alternatives for northeastern Illinois are the shallow aquifers and the Fox, Des Plaines, and Kankakee Rivers. Shallow groundwater resources and surface streams are hydraulically connected and withdrawal of water from the shallower systems is likely to have an effect on stream flow and wetlands. Consequently, conjunctive use scenarios must be examined through the collection of field data and computer modeling. In addition, shallow aquifers are vulnerable to surface-derived contaminants, and the intensity of urban development in the region has contributed to shallow groundwater quality degradation (e.g., Lake Calumet). The Water Resources Task Force of the Northeastern Illinois Planning Commission also recognized this need to quantify surface water and groundwater resource use, capacity, and quality in northeastern Illinois in recommending additional resources be provided to the Water Survey to support research and data collection.

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